1. Field of the Invention
This invention relates to a process and a device for producing a ceramic sintered body such as silicon nitride, etc.
2. Description of the Prior Art
Sintered bodies of ceramics, such as silicon nitride, silicon carbide, or alumina, are produced through a process comprising steps of mixing of starting powders, molding, and sintering it by using a powder metallurgy technique. In particular, sintered bodies of silicon nitride are widely used as materials for cutting tools or structural materials because of their high strength and toughness as well as superior thermal shock resistance and wear resistance. In recent years, ceramic sintered bodies of high quality have been desired that offer an improved strength with less variations in strength as application thereof to parts of an internal combustion engine of a vehicle.
In the production process for these ceramic sintered bodies, a grinder such as a ball mill is typically used to mix starting powders. Wet mixing is commonly used with a solvent such as alcohol to form a more uniform mixture. The slurry obtained as a result of the wet mixing is typically granulated by means of spray drying and is then molded through dry pressing and sintered. Alternatively, other available methods used in practice include injection with an addition of a large amount of plasticizer to granulated powder and wet molding to mold the slurry simultaneously with dehydration thereof by means of cast molding or the like.
The mixing method with a conventional grinder such as a ball mill imparts shock, friction, and shearing force to the starting powders by means of falling, colliding or rolling balls, rods, or pebbles in mixing, which causes the flocculation to be deflocculated or destroyed while the starting powders are ground. The starting powders are thus progressively dispersed in the solvent. In particular, pulverization of the starting powders by grinding is used positively as an approach to improve the strength of the sintered body in producing silicon nitride sintered bodies of high quality with a high strength. The imparted energies are, however, expended on motion or friction of a ground medium. This deteriorates the efficiency of mixing of the powders or dispersion thereof into the solvent, which is a primary object of grinding. Accordingly, a long time is inevitably required for mixing.
In addition, grinding of the starting powders in mixing will cause the variation in the particle diameter and the particle size distribution of the powders, depending on the mixing lot or mixing condition. This directly affects moldability of the powders and a powder packing structure of a molded compact, accompanying variation in density of the molded compact. This variation is a cause of deterioration or variations in strength and other properties of the final sintered bodies. Further, the ground medium may be worn during repeated collision or scrubbing with each other or with a pot. The tailings are contaminated in the starting powders as impurities. This contamination amount is increased as the mixing is performed over a prolonged period of time to ensure uniform mixture, deteriorating the quality of the sintered bodies.
On the other hand, a slurry of high powder content is preferable which contains a smaller amount of solvent and has a good fluidity when considering a molding rate in the wet molding of the slurry through, for example, the cast molding. A viscosity of the slurry is, however, increased and hence the fluidity thereof is deteriorated as the amount of the solvent is reduced. It may finally be difficult even to form a slurry. With this respect, grinding ability of the grinder has heretofore been improved or the grinding and mixing have been continued for a long time to obtain a slurry of high powder content. These methods are, however, disadvantageous in that the amount of contaminants from the grinding media is increased with the properties and the quality of the sintered bodies deteriorated. Similar problems occur in a case where the particles are formed from the slurry because there is also a need for increasing the powder content of the slurry by the energy-saving considerations.